Thanks a lot for the help, but the dudn is certainly not supposed to be removed. It is the directional derivative of u in the direction of n, the unit vector normal to the boundary Gamma of the domain Omega=[x_s,x_e]x[y_s,y_e]. This is why I tried to use the command PrincipalNormal. It, however, does not cope with Gamma when I define it by means of a LineSegments object. This is what I need to solve.